Table 2.

Competition affinities for different opioid receptors and ligands.

Receptor	Ligand	Ki (mean ± SEM) (nmol/L)	% Displacement
dre-μ	Met-ENK	684 ± 15	73.97 ± 1.29
	Leu-ENK	1,317 ± 166	58.51 ± 2.68
	β-END	186 ± 25	88.45 ± 2.03
	MEGY	204 ± 53	78.98 ± 3.73
	(d-Ala2)-MEGY	744 ± 27	66.72 ± 0.81
	(d-Ala2, Val5)-MEGY	3,645 ± 621	43.16 ± 3.39
	Morphine	187 ± 94	100
	Naloxone	10.65 ± 1.53	100
		Ki (nmol/L)		Reference
dre-δ1	β-END	36.6		29
	MEGY	427		21
	Morphine	22		29
dre-δ2	Met-ENK	45		30
	Leu-ENK	175
	MEGY	146		21
	Morphine	1,400		30
rn-μa	Met-ENK	1.80		5
	Leu-ENK	6.19
	MERF	0.37
	Morphine	7.48
rn-δb	Met-ENK	0.45
	Leu-ENK	0.37
	MERF	0.57
	Morphine	302
tg-μc	Met-ENK	70.7		32
	Leu-ENK	117
	β-END	55.9
tg-δc	Met-ENK	24.9
	Leu-ENK	198
	β-END	284

This table summarizes the results obtained in competition binding assays (Ki and % displacement) using [³H]-diprenorphine on membrane homogenates from HEK293 cells that stably express the μ opioid receptor from zebrafish. The results of other competition binding studies for the zebrafish δ opioid receptors dre-δ1 and dre-δ2, prototypical mammalian δ and μ receptors rn-oprm1 (rn, Rattus norvegicus [rat]) and rn-oprm1 and the amphibian μ and δ receptors tg-oprm1 and tg-oprd1 (tg, Taricha granulosa [newt]) have also been included for comparison.

^aIn this case, the radioligand used was the μ-selective peptidic analog [³H]-DAMGO and not [³H]-diprenorphine.

^bIn this case, the radioligand used was the δ-selective peptidic analog [³H]-DPDPE and not [³H]-diprenorphine.

^cIn this case, the radioligand used was the nonspecific antagonist [³H]-naloxone and not [³H]-diprenorphine.